Gestational exposure to ambient air pollution with black carbon and birthweight: A register-based nationwide birth cohort in Denmark.

Air pollution has been linked to reduced birthweight, but the association with black carbon (BC) remains insufficiently explored. This study examined the association of gestational exposure to BC with birthweight outcomes. We included data on all singleton births in Denmark between 2004 and 2016 (n = 690,898). BC and fine particulate matter (PM2.5) were estimated by high-resolution air pollution models based on maternal residential addresses during pregnancy. Linear and Poisson regression models were employed to assess the associations of BC with birthweight, low birth weight (LBW), and small for gestational age (SGA), adjusting for spatiotemporal and individual-level covariates. Effect modification by gender, mothers' age and socio-economic status (SES) were examined. Gestational exposure to BC was associated with birthweight and SGA risk: interquartile range (0.34 μg/m3) increase in BC was associated with -9 g (95% CI: 10, -7) decrease in birthweight, a relative risk (RR) of 1.02 (1.01, 1.03) for SGA, and a RR of 1.01 (1.00, 1.03) for LBW. After further adjustment for PM2.5, the associations remained significant for birthweight, were borderline significant for SGA, and null for LBW. Associations of BC with birthweight were stronger in young mothers, in mothers with low SES, and in mothers with higher BMI. In this large nationwide study, we show that gestational exposure to BC is associated with birthweight, persisted after adjusting PM2.5, adding important new evidence supporting regulation of BC in addition to PM2.5.